Molecular alignment of polymer liquid crystals in shear flows. 1. Spectrographic birefringence technique, steady-state orientation, and normal stress behavior in poly(benzyl glutamate) solutions

Abstract

We describe results of an experimental investigation into the orientation state of liquid crystalline solutions of poly(benzyl glutamate) under shear flow and how the microscopic structure relates to the macroscopic mechanical rheological behavior. The technique of flow birefringence was used to study the degree of molecular orientation. A spectrographic flow birefringence apparatus is described that eliminates ambiguities associated with multiple orders of retardation in birefringence measurements. The birefringence observed in textured solutions under shear flow is always less than that measured in quiescent, defect-free monodomains of the solutions. At low shear rates, the birefringence is roughly constant and in the range of 53-63% of that observed in a monodomain; there is no evidence of a low-orientation, "piled polydomain" structure. At high shear rates, the birefringence is again roughly constant and around 90% of the monodomain value. The transition between lowand high-orientation states as a function of shear rate is closely correlated with changes in sign of the first normal stress difference of these solutions, leading us to identify it as a manifestation of a transition between regimes of director tumbling at low shear rates and flow alignment at high shear rates. These observations are compared qualitatively and quantatively with predictions of the nonlinear Doi molecular model for textureless samples [Larson, R. G. Macromolecules 1990,23, 3983] and the linear Larson and Doi tumbling polydomain model for textured samples [Larson, R. G.; Doi, M. J. Rheol. 1991, 35, 539]. An accompanying paper considers transient flow phenomena at low shear rates.

Optical and Mechanical Rheometry of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers with Varying Pendant Groups

“…A second interpretation of these observations is that fractional A s data may arise from polarization state mixing; 26 however, we have found trends similar to those in Figure 4 using a different optical technique. In particular, we have compared A s + (t,γ ) data with the growth in unpolarized light transmission ("transmission") as measured at a wavelength of 700 nm with no analyzer and relative to the intensity transmitted by an isotropic sample.…”

“…Therefore, we have adopted a spectrographic birefringence technique. 26,27 To the best of our knowledge, spectrographic birefringence has not been previously applied to in-situ measurements on flowing TLCPs, despite the power of this method. The optical train for our spectrographic birefringence setup is shown schematically in Figure 1.…”

Effects of Wavelength on Strain-Induced Birefringence of Polymers

“…The optical train is a standard one for measurement of birefringence with a spectrometer. 5 The polarization axes of two polarizing prisms were set at angles n/4 and -n/4 or n/4 and n/4 with respect to the elongation direction. The former is referred to as the crossed polarizers (1_), and the latter as the parallel (//).…”

“…5 Liquid crystalline systems are those difficult ones exhibiting strong birefringence. A. effects seem to be carefully taken into account in some cases 5 but not so much in others. 13 …”

“…3 Recently the A. dependence of birefringence is reported for some polymeric liquid crystalline systems in regard to the measuring method of birefringence with white light and a spectrometer. 5 Apart from these few studies, the effect of wavelength on the birefringence has not been studied in detail.…”